Liquid crystal displays (LCDs) are commonly used in cell phones, personal digital assistants (PDAs), portable music players, laptop computers, desktop monitors, and television applications. One embodiment of the present invention deals with a color or monochrome, transmissive LCD that requires backlighting, where the backlight may use one or more LEDs emitting white or colored light. The LEDs are distinguished from laser diodes in that the LEDs emit incoherent light.
In many small displays, such as for cell phones, it is important that the display and backlight be thin. In such small backlights, one or more LEDs are optically coupled to or near the edges of a solid transparent lightguide (also referred to as a waveguide). Light in the lightguide leaks out a top surface of the lightguide to illuminate a back surface of a liquid crystal display (LCD) panel. In such small backlights, the LCD panel does not overlie the LEDs since the LEDs would appear as a bright spot or, if the LEDs were only side-emitting, the LEDs would appear as a dark spot. Such edge-coupling to a solid lightguide is not practical for a large backlight due to the attenuation of light through the lightguide, making the center area much darker than the edge areas and/or reducing the total efficiency and light output.
For large backlights, such as for televisions, LEDs are typically distributed on the bottom surface of a reflective backlight box. The light from the array of LEDs is mixed in the box, to create a substantially uniform light emission, by the light rays overlapping and the light being reflected off the walls of the box. Such a backlight box is rigid. Large backlight boxes are difficult to handle and are expensive to manufacture.
What is needed is a backlight that can be scaled for large applications, yet does not have the drawbacks of large backlights formed using LEDs within a rigid backlight box.